Electrophilicity versus electrofugality of tritylium ions in aqueous acetonitrile

Article abstract of DOI:10.1002/chem.200902670

First-order rate constants k_w for the reactions of a series of donorsubstituted triphenylmethylium (tritylium) ions with water in aqueous acetonitrile have been determined photometrically at 20°C using stopped-flow and laser-flash techniques. T

Full text of DOI:10.1002/chem.200902670

DOI: 10.1002/chem.200902670
Electrophilicity versus Electrofugality of Tritylium Ions in Aqueous
Acetonitrile
Markus Horn and Herbert Mayr*[a]
Dedicated to Prof. Rudolf Knorr on the occasion of his 75th birthday
Abstract: First-order rate constants kw
for the reactions of a series of donor-
substituted triphenylmethylium (trity-
lium) ions with water in aqueous aceto-
nitrile have been determined photo-
metrically at 208C using stopped-flow
and laser-flash techniques. The rate
constants follow the linear free energy
culations of tritylium ions and the cor-
responding triarylmethanols and 1,1,1-
triarylethanes have been performed at
linearly with the calculated gas-phase
hydroxide affinities, and the slope of
this correlation shows that the differen-
ces in carbocation stabilities in the gas
phase are attenuated to 66% in solu-
tion. Mono- and bis(dimethylamino)-
substituted derivatives deviate from
this correlation; their pKR+ values are
higher than expected from their calcu-
lated gas-phase hydroxide affinities,
which is explained by the extraordinary
solvation of unsymmetrically amino-
the MP2(FC)/6-31+G
31G(d,p) level. The calculated gas-
ACHTUNGTREN(NUGN 2d,p)//B3LYP/6-
AHCTUNGTRENNUNG
phase hydroxide and methyl anion af-
finities of the tritylium ions correlate
linearly with a slope of unity, indicating
that the relative anion affinities do not
depend on the nature of the anion. The
pKR+ values of the methyl- and meth-
relationship logk
reactivities kw of the methyl- and meth-
oxy-substituted tritylium ions towards
ACHTUNGTRENNUNG(208C)=sCAHTUNGTRENN(UGN N+E). The
ACHTUNGTRENNUNG
water correlate linearly with the corre-
sponding pKR+ values with a Leffler–
Hammond coefficient a=dDGꢀ/dDG0
of 0.62. The amino-substituted com-
pounds react more slowly than expect-
ed from the correlation of the less sta-
bilized systems. Quantum chemical cal-
ACHTUNGTNERoUNGN xy-substituted triACHTUNGTREGtNNUN ylACHTNUGTRNEiNUGN um ions correlate
substituted tritylACTHNUTRGNEUGNium ions. Complete
free-energy profiles for the solvolyses
of substituted trityl benzoates in 90:10
(v/v) acetonitrile/water have been con-
structed.
Keywords: free-energy profiles · in-
trinsic barriers · kinetics · linear
free-energy relationships · tritylium
ions
Introduction
In order to elucidate the origin of this unique breakdown
of a rate–equilibrium relationship, we have now investigated
the electrophilic reactivities of differently substituted trityl-
Stabilities (more precisely, Lewis acidities) of carbocations[1]
are commonly associated with the rates of their formation in
solvolysis reactions and the rates of their reactions with nu-
cleophiles. In the preceding article we have reported that
the tris(p-methoxy)tritylium ion and the p-(dimethylamino)-
tritylium ion are formed with almost equal rates from the
corresponding trityl acetates and benzoates, despite the 103-
fold higher stability (pKR+) of the latter carbocation.
AHCTUNGTREGiNNUN um ions in aqueous acetonitrile, that is, the same solvent as
used for the solvolysis studies in the preceding article.[2]
These investigations were accompanied by quantum chemi-
cal calculations.
Early studies on the electrophilic reactivities of tritylium
ions focused on stabilized amino- and methoxy-substituted
species.[3–10] In particular, malachite green, (Me2N)2Tr +, and
derivatives thereof, were subjects of many investigations.[3]
Rate constants for the reactions of (Me2N)2Tr +, p-nitromala-
chite green, and crystal violet, (Me2N)3Tr +, with water, hy-
droxide, and cyanide in aqueous solution[4] were the founda-
tion of Ritchieꢀs well-known constant selectivity relation-
[a] M. Horn, Prof. Dr. H. Mayr
Department Chemie, Ludwig-Maximilians-Universitꢁt Mꢂnchen
Butenandtstr. 5–13, 81377 Mꢂnchen (Germany)
Fax : (+49)89-2180-77717
ship: logACTHNUTRGNEUNG
(k/k0)=N+.[5] Bunton and Hill studied the kinetics
of the reactions of the tris(p-methoxy)tritylium cation with
water and hydroxide in aqueous solution. While Buntonꢀs
work concentrated on the salt effects for these reactions,[6]
E-mail: Herbert.Mayr@cup.uni-muenchen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902670.
7478
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Chem. Eur. J. 2010, 16, 7478 – 7487

FULL PAPER
Hillꢀs studies of kinetic isotope effects showed that the first
step, the addition of water to the carbocation, rather than
the subsequent proton transfer, is the rate-determining
step.[7] Taft reported that the rates of the reactions of me-
thoxy- and dimethylamino-substituted tritylium ions with
water are not closely correlated with their thermodynamic
stabilities,[8] and suggested separation into families. Later in-
vestigations confirmed Taftꢀs experimental results, but dem-
onstrated that the deviations from the linear logkw/pKR+
correlation are marginal when an extended series of com-
pounds is considered.[5c,9] Reactivities of less stabilized sys-
tems were studied by McClelland, who used laser-flash tech-
niques for the in situ generation of the carbocations.[9,10]
cases water was used in large excess, pseudo-first-order rate
laws were obeyed, as illustrated in Figure 1. The obtained
curves were fitted to the mono-exponential function At =
A0eꢀkt +C by the method of least squares.
Results and Discussion
Kinetics: Rate constants kw for the reactions of water with
the tritylium ions (Scheme 1) listed in Table 1 were deter-
mined at 208C in aqueous acetonitrile by photometric moni-
toring of the decays of the tritylium ions, which have absorp-
tion maxima between 420 and 504 nm. Different techniques
have been utilized for this purpose.
Figure 1. Absorbance decay of (MeO)Tr+ at 472 nm. The carbocation
was generated from the corresponding acetate (c0 =1.00ꢄ10ꢀ4 molLꢀ1
)
using a laser pulse (7 ns, 266 nm, 60 mJ) in 50:50 (v/v) acetonitrile/water,
208C.
Aqueous solutions of the amino-substituted systems
ꢀ
ꢀ
ACTHNUTRGNEUNG
(Me2N)Tr+BF4 and (Me2N)(MeO)Tr+BF4 in acetonitrile
did not decolorize completely. When small amounts of tetra-
n-butylammonium acetate or benzoate were added to trap
the generated protons, complete consumption of these car-
bocations was achieved. The tritylium ions (Me2N)2Tr + and
(Me2N)3Tr + are so stable in aqueous solution that even the
addition of large amounts of carboxylates did not lead to
noticeable changes in the carbocation concentrations. For
this reason we constrained our studies to systems with pKR+
<5.
Scheme 1. Reactions of tritylium ions with water.
Table 1. Tritylium ions with corresponding pKR+ values and electrophili-
ACHTUNGTRENNUNGcity parameters E.
Are the rate constants for the reactions of tritylium ions
with water (kw) affected by the presence of carboxylate
ions? It has been reported that tertiary amines, such as 1,4-
R1, R2, R3[a]
Abbreviation
pKR+
E[c]
0.51
[b]
H, H, H
Me, H, H
Me, Me, H
Me, Me, Me
MeO, H, H
Tr+
ꢀ6.63
ꢀ5.41
ꢀ4.71
ꢀ3.56
ꢀ3.40
ꢀ1.24
0.82
MeTr+
Me2Tr +
Me3Tr +
ꢀ0.13
ꢀ0.70
ꢀ1.21
ꢀ1.87
ꢀ3.04
ꢀ4.35
ꢀ7.93
ꢀ7.98
ꢀ10.29
ꢀ11.26
diazabicycloACTHNUGTRNEUNG[2.2.2]octane (DABCO), triethylamine, or qui-
nuclidine, act as general base catalysts for the addition of
water to tritylium ions in pure water,[11] whereas general
base catalysis by acetate has not been detected.[6,7] To inves-
tigate the influence of carboxylate ions on the reaction ki-
netics in aqueous acetonitrile, the consumption rates of
(MeO)Tr+
MeO, MeO, H
MeO, MeO, MeO
Me2N, H, H
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
G
3.88[d]
Me2N, MeO, H
Me2N, Me2N, H
Me2N, Me2N, Me2N
N
E
4.86[e]
6.94[e]
9.39[e]
ꢀ
(MeO)3Tr +BF4 in 90:10 (v/v) acetonitrile/water have been
studied in the presence of variable amounts of (nBu)4N+
AcOꢀ and (nBu)4N+BzOꢀ at 258C. Figure 2 shows that an
increase of the concentration of AcOꢀ or BzOꢀ accelerated
the mono-exponential decay of (MeO)3Tr + linearly. Similar
[a] For the location of the substituents see Scheme 1. [b] From refer-
ence [9]. [c] Empirical electrophilicity parameters from reference [24].
[d] From reference [8]. [e] From reference [5c].
ꢀ
experiments were performed with (Me2N)Tr+BF4 , and
Table 2 gives an overview of all parameters obtained from
regression lines as depicted in Figure 2.
The more reactive ions, Tr+, MeTr+, Me2Tr +, Me3Tr +,
and (MeO)Tr+, were generated in situ by laser-flash photol-
ysis of the corresponding trityl acetates in aqueous acetoni-
trile. The other tritylium ions were introduced as tetrafluo-
The small slopes for the reactions of (Me2N)Tr+ with
AcOꢀ and BzOꢀ (entries 7–9) indicate that only high con-
centrations of carboxylate ions have a significant effect on
the rate of consumption of (Me2N)Tr+. As only 4.6 to 8.7
equivalents of carboxylate (c=0.15 to 4.9 mmolLꢀ1) were
ACHTUNGTRENNUNG ACHTUNGTRENNUNGborate salts, and their reactions were followed by stopped-
Chem. Eur. J. 2010, 16, 7478 – 7487
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7479

H. Mayr and M. Horn
of Figure 2 and Table 2 is given in the section “Common ion
return of carboxylate anions?”.
The presence of dications like [(HMe2N)Tr]2+, i.e., N-pro-
tonated dimethylamino-substituted tritylium ions, has been
excluded in neutral aqueous solutions.[11c] Because in this
work dimethylamino-substituted systems have been studied
in the presence of carboxylate ions, i.e., under slightly basic
conditions, the contribution of dicationic species could be
neglected.
Quantum chemical calculations: A calculated geometry for
the parent tritylium cation has previously been reported.[12a]
Aizman, Contreras, and Pꢅrez have performed DFT calcula-
tions of substituted tritylium ions at the B3LYP/6-31G(d)
level in order to determine their Parr electrophilicity param-
eters.[12b] Because neither geometries nor energies have been
reported, we have now optimized the geometries of trityl-
ꢀ
Figure 2. Plot of kobs for the decay of the absorbance of (MeO)3Tr +BF4
(c0 =3 ꢄ 10ꢀ5 molLꢀ1) in 90:10 (v/v) acetonitrile/water versus the concen-
tration of added tetra-n-butylammonium carboxylate, l=484 nm, 258C.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
298.15 K have been calculated for all minima from unscaled
vibrational frequencies obtained at the same level, and com-
bined with single-point energies on the MP2(FC)/6-31+G-
Table 2. Kinetics of the consumption of tritylium ions in aqueous aceto-
nitrile with different additives in excess at 258C.
Entry Electrophile Additive Solvent[a] Slope
[Mꢀ1 sꢀ1
Intercept
[sꢀ1
AHCTUNTGREUNN(GN 2d,p) level to yield enthalpies H298 and free energies G298.
]
]
For tritylium ions carrying two or three p-methoxy groups,
and alcohols or ethanes carrying at least one p-methoxy
group, different conformations have been considered (see
Supporting Information). The structural parameters of the
energetically best conformers of the carbocations are sum-
marized in Table 4.[13]
1
2
3
4
5
6
7
8
9
N
4.82
6.85
4.96
4.34
7.32
90AN10W 2.32ꢄ104
90AN10W 2.62ꢄ105
90AN10W 3.52ꢄ10ꢀ1
[d]
–
3.55ꢄ10ꢀ3
3.66ꢄ10ꢀ3[e]
3.48ꢄ10ꢀ3
3.55ꢄ10ꢀ3
When an ion contains three equal para-substituents (H,
Me, MeO, or Me2N), the dihedral angle decreases slightly as
the electron-donating ability of the substituent increases
(33.6, 33.1, 32.9, and 32.48, respectively) and the bond
lengths remain constant (1.45 ꢆ). For tritylium ions with dif-
ferently substituted rings, the rings are distorted out of the
plane to a different extent. The better the electron-donating
ability of the para-substituent, the smaller the dihedral
angle of the corresponding ring, and the shorter the distance
between the ring and the central carbon. For (Me2N)-
10
[a] The solvent is given in vol%, AN=acetonitrile, W=water.
90AN10W=90:10 (v/v) acetonitrile/water, etc. [b] The counterion was
BF4ꢀ. [c] The counterion was (nBu)4N+. [d] Not reliable, because inter-
cept !kobs. [e] At 208C.
employed to achieve complete consumption of (Me2N)Tr+
and (Me2N)ACHTUNGTRENNUNG
(MeO)Tr+ in the kinetic experiments at 208C,
we neglected their influence on the rate constants kw, which
(MeO)Tr+, the smallest dihedral angle is calculated for the
ACHTUNGTRENNUNG
are summarized in Table 3. An interpretation of the slopes
ring carrying the dimethylamino group (26.58), an intermedi-
ate angle for the methoxy-substituted ring (32.88), and the
unsubstituted ring is twisted by
40.08. The decreasing resonance
Table 3. First-order rate constants kw for the reactions of tritylium ions with water in aqueous acetonitrile[a] at
208C.[b]
contribution from the dimeth
ACHTUNGTRENNUNGyl-
AHCTUNGTERGaNNUN mino- over the methoxy-sub-
stituted to the unsubstituted
ring is also indicated by the cor-
kw [sꢀ1
60AN40W
]
90AN10W
80AN20W
50AN50W
33AN66W[c]
Tr +
1.19ꢄ105
2.44ꢄ104
7.85ꢄ103
2.77ꢄ103
1.17ꢄ103
4.16ꢄ101
3.73
1.58ꢄ105
3.60ꢄ104
9.35ꢄ103
3.01ꢄ103
1.43ꢄ103
5.61ꢄ101
4.78
1.69ꢄ105
4.29ꢄ104
9.84ꢄ103
3.17ꢄ103
1.75ꢄ103
5.47ꢄ101
4.93
1.62ꢄ105
4.08ꢄ104
9.89ꢄ103
2.83ꢄ103
1.73ꢄ103
5.81ꢄ101
4.88
1.6ꢄ105
3.7ꢄ104
1.1ꢄ104
3.6ꢄ103
1.4ꢄ103
8.6ꢄ101
1.0ꢄ101
–
ꢀ
responding bond lengths C2 C3,
MeTr+
Me2Tr +
Me3Tr +
which increase from 1.43 to
1.45 to 1.47 ꢆ, respectively.
(MeO)Tr+
ACHTUNGTRENNUNG
Though these calculations
refer to the gas phase, spectro-
scopic investigations in solution
confirm these structural assign-
ments. The extraordinarily high
resonance contribution of the
ACHTUNGTRENNUNG
N
2.57ꢄ10ꢀ3
1.53ꢄ10ꢀ3
3.43ꢄ10ꢀ3
1.97ꢄ10ꢀ3
3.77ꢄ10ꢀ3
2.16ꢄ10ꢀ3
3.77ꢄ10ꢀ3
2.14ꢄ10ꢀ3
A
N
–
[a] Solvents are given in vol%, AN=acetonitrile, W=water. [b] Note that the rate constants in this table refer
to a different temperature than those in Table 2. [c] From ref. [10a].
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Electrophilicity versus Electrofugality of Tritylium Ions
Table 4. Structural parameters[a] of tritylium ions, B3LYP/6-31G
ACTHNUGTENR(NUG d,p).
FULL PAPER
Gas-phase hydroxide and
methyl anion affinities (Table 5)
have been calculated according
to Equations (1) and (2), and
are shown to correlate linearly
(Figure 4).
ꢀ
Dihedral angle C1C2C3C4 [8]
Bond length C2 C3 [ꢆ]
Tr +
33.6 (H)[b]
34.4 (H)
35.3 (H)
33.1 (Me)
36.1 (H)
38.4 (H)
32.9 (MeO)
38.3 (H)
1.45 (H)
MeTr+
Me2Tr +
Me3Tr +
31.2 (Me)
32.2 (Me)
1.45 (H)
1.45 (H)
1.45 (Me)
1.46 (H)
1.46 (H)
1.45 (MeO)
1.46 (H)
1.44 (Me)
1.45 (Me)
R3Cþ þ OHꢀ ! R3CꢀOH ð1Þ
(MeO)Tr+
ACHTUNGTRENNUNG
28.0 (MeO)
30.5 (MeO)
1.43 (MeO)
1.44 (MeO)
R3Cþ þ Meꢀ ! R3CꢀMe
ð2Þ
ACHTUNGTRENNUNG
E
24.0 (Me2N)
32.8 (MeO)
28.7 (Me2N)
1.42 (Me2N)
1.45 (MeO)
1.43 (Me2N)
The slope of unity implies
that structural variations of the
tritylium ions affect their affini-
ties toward Meꢀ and OHꢀ to
the same extent. A similar be-
havior has previously been re-
T
(MeO)Tr+ 40.0 (H)
26.5 (Me2N) 1.47 (H)
1.47 (H)
1.43 (Me2N)
C
41.9 (H)
A
32.4 (Me2N)
1.45 (Me2N)
[a] The values are assigned to the rings with the substituents in parentheses. [b] In reference [14] a value of
32.48 was determined for tritylium perchlorate by X-ray diffraction.
Table 5. Theoretical gas-phase hydroxide and methyl anion affinities
DH298, MP2(FC)/6-31+G(2d,p)//B3LYP/6-31G
(d,p).[a]
dimethylamino group can be directly observed in the
G
ACHTUNGTRENNUNG
ꢀ
13C NMR spectrum of (Me2N)
G
DH298 [Eq. (1)] [kJmolꢀ1
]
DH298 [Eq. (2)] [kJmolꢀ1
]
The two ortho-carbon atoms, as well as the two meta-carbon
atoms of the dimethylamino-substituted ring, are not iso-
chronous due to the high rotational barrier of this ring.
As neither of the two signal pairs coalesces in acetonitrile
solution at 708C (100 MHz), the interconversion barrier for
these carbon atoms must be higher than 75 kJmolꢀ1. The
equivalence of the corresponding carbon signals in the other
two phenyl rings even at ambient temperature indicates a
fast rotation of these rings on the NMR timescale. The de-
tails of the dynamic behavior of tritylium cations have previ-
ously been investigated by several groups.[15]
Tr +
ꢀ715.54
ꢀ704.42
ꢀ694.23
ꢀ684.90
ꢀ688.99
ꢀ667.15
ꢀ649.64
ꢀ648.75
ꢀ883.72
ꢀ871.61
ꢀ862.00
ꢀ852.30
ꢀ857.22
ꢀ835.36
ꢀ817.64
ꢀ816.71
ꢀ799.91
ꢀ774.20
ꢀ740.66
MeTr+
+
Me2
Me3Tr +
AHCTUNGTRENNUNG
(MeO)Tr+
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHUTNGREN(NUG Me2N)ACHTUGNTRENNNUG
(Me2N)2
+
ꢀ605.42
ꢀ574.12
[a] For total energies and thermochemical corrections needed for the cal-
culations see the Supporting Information.
Figure 4. Correlation of theoretical Meꢀ and OHꢀ affinities (DH298 in
kJmolꢀ1), MP2(FC)/6-31+G (d,p), slope=1.00, R2 =
ACTHUNTGRNENUG(2d,p)//B3LYP/6-31GAHCTUNGTRENNGUN
1.00.
ported for benzhydrylium ions.[16] In each of the three sub-
AHCTNUTGREGsNNUN eries MexTr, (MeO)xTr, and (Me2N)xTr, the substituent
effect on the anion affinity decreases as one goes from
mono- to di- and trisubstituted systems (Figure 5). Here, the
common saturation effects in multisubstituted systems are
enforced by the propeller-like conformations of the tritylium
ions, which allow the first substituent to stabilize the carbo-
ACTHNUTRGNEUNG
Figure 3. Part of the 13C NMR spectrum (150 MHz) of (Me2N)(MeO)Tr+
BF4ꢀ in CDCl3 at 278C.
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7481

H. Mayr and M. Horn
substituents in the gas phase are reduced to 66% in aqueous
solution. A value of 72% has been reported for benzhydryli-
um ions.[16]
For the construction of the rate–equilibrium relationship
in Figure 7, we have complemented the kw values of Table 1
with the rate constants for (Me2N)2Tr + and (Me2N)3Tr +,
Figure 5. Plot of relative OHꢀ affinities (DH298 in kJmolꢀ1
0 kJmolꢀ1) versus the number of identical substituents in ArxPh(3ꢀx)C+,
MP2(FC)/6-31+G(2d,p)//B3LYP/6-31G(d,p).
, =
Tr+
A
ACHTUNGTRENNUNG
cation more efficiently by planarizing the donor-substituted
ring and squeezing the less electron-donating rings out of
plane.
Figure 7. Plot of logkw (50% aqueous acetonitrile, 208C) versus pKR+
(from Table 1); logkw of (Me2N)2Tr + and (Me2N)3Tr + =ꢀ3.97 and ꢀ4.94,
respectively (from refs. [3a,11c], see text); the open circles have been
omitted from the linear regression, slope=ꢀ0.62, R2 =0.99.
Linear free-energy relationships: The comparison of hydrox-
ide affinities in solution (pKR+) with the corresponding gas-
phase data shows a good correlation for the methyl- and
methoxy-substituted systems as well as the tris(dimethylami-
no)-substituted tritylium ion. However, the unsymmetrical
dimethylamino-substituted systems (Me2N)Tr+, (Me2N)2Tr +,
which have previously been reported in the literature. Cigꢅn
has determined the rate constant of the reaction of
(Me2N)2Tr + with water in pure water at 208C as 1.08ꢄ
10ꢀ4 sꢀ1.[3a] Ritchie reported a rate constant of 1.94ꢄ10ꢀ5 sꢀ1
for (Me2N)3Tr + at 258C.[11c] With a value of 73.3 kJmolꢀ1 for
the enthalpy of activation (calculated with data taken from
ref. [11c]), one can calculate a rate constant of 1.15ꢄ10ꢀ5 sꢀ1
at 208C on the basis of the Eyring equation.
Table 3 shows that the nucleophilic reactivity of water in
aqueous acetonitrile remains almost constant as the amount
of water exceeds 20 vol%. This observation is in agreement
with McClellandꢀs[9] and our previous observations[17] that
carbocations are trapped with almost equal rates in different
acetonitrile/water mixtures containing 20 to 100% water.
We can, therefore, assume that the kw values determined by
Cigꢅn and Ritchie in pure water[3a,11c] also hold for 50:50 (v/
v) acetonitrile/water, and we included them in the correla-
tion of Figure 7. Like the ionization rate constants of trityl
acetates,[2] only the electrophilicities of the methyl- and
and (Me2N)ACHTUNGTRENNUNG
(OMe)Tr+ (open circles in Figure 6) are more
stable in solution than expected on the basis of their gas-
phase hydroxide affinities. These deviations indicate excep-
tionally high solvation enthalpies of the mono- and diamino-
substituted tritylium ions, which may account for the high
intrinsic barriers for the formation and reactions of these tri-
tylium ions (see below). The slope of the line drawn in
Figure 6 (0.66) suggests that the stabilizing effects of the
meth
on their Lewis acidities (pKR+). The unsymmetrical dimeth-
ylamino-substituted tritylium ions react more slowly than
ACHTUNGTERNoNUNG xy-substituted tritylium ions (logkw) depend linearly
A
A
expected from their thermodynamic stabilities in aqueous
solution, indicating higher intrinsic barriers for the reactions
of these systems. In line with these findings, highly reso-
nance-stabilized carbocations have previously been reported
to show low intrinsic reactivities.[18]
Figure 6. Correlation of DG0 (=2.303RTpKR+) of water attack at trityl-
+
Hammett analysis: Hammett–Brown parameters sp were
ACHTUNGTRENNUNG
ium ions versus calculated OHꢀ affinities (gas phase, DG298, MP2(FC)/6-
designed for reactions involving a positively-charged center
31+GACHTUNGTRENNUNG(2d,p)//B3LYP/6-31GACHTUTGNREN(NUGN d,p)); the open circles have been omitted
from the linear regression, slope 0.66.
in conjugation to the substituents under consideration.[19]
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Electrophilicity versus Electrofugality of Tritylium Ions
FULL PAPER
Figure 8 shows that the four symmetric systems Tr, Me3Tr,
(MeO)3Tr, and (Me2N)3Tr correlate perfectly linearly with
Ssp+. Unsymmetrically substituted systems deviate, and the
Figure 9. Plot of logkw versus E-parameters of tritylium ions; for kw of
(Me2N)2Tr + and (Me2N)3Tr +: see text; 50:50 (v/v) acetonitrile/water,
208C, R2 =0.99.
+
Figure 8. Plot of logkw (50% aqueous acetonitrile, 208C) versus Ssp
The quality of the correlation shown in Figure 9 corrobo-
rates the reliability of the previously reported E-parame-
ters.[24] Moreover, if Figure 9 had been employed to deter-
mine the nucleophilicity parameters of 50AN50W (50:50 (v/
v) acetonitrile/water), values of N=5.32 and s=0.86 would
have been obtained, close to the parameters derived from
reactions with benzhydrylium ions (N=5.05 and s=0.89).[17]
The internal consistency of our reactivity parameters is thus
demonstrated.
(sp+ =0 (H), ꢀ0.31 (Me), ꢀ0.78 (OMe), and ꢀ1.70 (NMe2) from
ref. [22]; for kw of (Me2N)2Tr + and (Me2N)3Tr +: see text); the regression
line is drawn through the four symmetrically substituted systems Ar3Tr +,
1=1.99, R2 =1.00.
magnitude of the deviations increases with increasing elec-
tron-donating ability of the p-substituents. A similar behav-
ior has been found for the correlation between the ioniza-
+
[2]
tion rates of trityl esters and Ssp
.
Many examples have
shown that in the case of multiple ring substitution in di-
and triarylcarbenium ions, s+ parameters are non-addi-
tive,[9,20] for reasons which have been discussed previously.[20]
Comparison of electrofugality and electrophilicity: It is gen-
erally assumed that highly stabilized carbocations are gener-
ated rapidly in solvolysis reactions, and react slowly with nu-
cleophiles. Recently, we have reported that the inverse rela-
tionship between electrofugality (rate of formation of R+ in
a heterolytic process) and electrophilicity (rate of reactions
of R+ with nucleophiles), which holds for nonstabilized and
slightly stabilized benzhydrylium ions, breaks down for
Electrophilicity parameters of tritylium ions: Equation (3),
in which E is an electrophile-specific reactivity parameter,
and s and N are nucleophile-specific parameters, was de-
signed to correlate bimolecular rate constants k2 for electro-
phile–nucleophile combinations.[21]
amino-substituted benzhydrylium ions.[25]
A comparably
poor correlation between electrophilicity and electrofugality
for tritylium ions is shown in Figure 10.
log k2 ð20 ꢁCÞ ¼ sðN þ EÞ
ð3Þ
As in the series of benzhydrylium ions, electrophilicity is
inversely correlated with electrofugality for methyl- and me-
thoxy-substituted systems, whereas amino-substituted sys-
tems ionize much more slowly than expected from the rates
of their reactions with nucleophiles. The same argument
that has been used to rationalize the deviation of the amino-
substituted tritylium ions from the logkion/pKR+ correlation
(Figure 12 in ref. [2]), that is, late development of the reso-
nance stabilization by the amino group on the reaction coor-
dinate of the ionization process, can be used to explain the
A set of benzhydrylium ions and quinone methides were
used as reference electrophiles for determining the N and s
parameters of a large number of s-, n-, and p-nucleo-
philes.[23] Because steric effects are not specifically included,
Equation (3) only provides reliable predictions of rate con-
stants for reactions of nucleophiles with carbocations and
Michael acceptors if bulky systems are excluded. It has been
discussed that reactions of tritylium ions can only be treated
by Equation (3) if nucleophiles with negligible steric re-
quirements, for example, primary amines and alcohols or hy-
dride donors, are considered.[24] Figure 9 shows a good linear
correlation of logkw with the electrophilicity parameters E
of tritylium ions (Table 1), which have previously been de-
rived[24] from the rate constants for their reactions with
water and n-PrNH2 taken from the literature.
correlation in Figure 10. Because these deviations turn up in
[2]
each of the correlations logkion/pKR+
,
logkw/pKR+
(Figure 7) and logkion/logkw (Figure 10), they cannot be due
to errors in one of the data sets.
Chem. Eur. J. 2010, 16, 7478 – 7487
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7483

H. Mayr and M. Horn
ꢀd½Rþꢂ=dt ¼ ½Rþꢂðkw þ kOH½OHꢀꢂ þ kꢀion½Bꢂ þ kcat½BꢂÞꢀkion½RꢀBꢂ
ð5Þ
While the intercepts of the lines shown in Figure 2 can be
assigned unambiguously to kw (4.71 sꢀ1 in the case of
(MeO)3Tr + in 90AN10W, average of entries 1, 3, 4 of
Table 2), the interpretation of the slopes is less straightfor-
ward.
For the reaction of (MeO)3Tr + with hydroxide in pure
water, a second-order rate constant of kOH =8200mꢀ1 sꢀ1 has
been reported,[6] which is considerably smaller than the
value of
k
OH =2.62ꢄ105 mꢀ1 sꢀ1 in 90AN10W (Table 2,
entry 6). This difference can be explained by the better sol-
vation of hydroxide in water than in acetonitrile. However,
at the low concentrations of hydroxide present under the
Figure 10. Plot of logkw (208C) versus logkion (acetates, 258C, from
ref. [2]), 90:10 (v/v) acetonitrile/water; logkw for (Me2N)2Tr + in
90AN10W has been calculated by dividing 1.08ꢄ10ꢀ4 (kw in 50AN50W)
conditions of these experiments, k
[OHꢀ] appears to be
OHACHTUGNTRENNNUG
negligible, because comparison of entries 1, 3, and 4 of
Table 2 shows that the acceleration by DABCO is smaller
than that by AcOꢀ and BzOꢀ, in spite of the much higher
basicity of DABCO, which must lead to higher OHꢀ concen-
trations.
by 1.36 (average ratio k
ACHTUNGTRENNUNGium ions in Table 3).
wACHUTNGTRENG(NU 50AN50W)/kwACHTUNTGREN(NUNG 90AN10W) for all other trityl-
Common ion return of carboxylate anions? In order to de-
termine the rate constants kw for the reactions of dimethyla-
mino-substituted tritylium ions in aqueous acetonitrile, we
had to add AcOꢀ and BzOꢀ as proton traps to suppress reio-
nization of the generated triarylmethanols. We will now ana-
lyze the origin of the rate enhancement by these carboxylate
ions, which was described in Figure 2 and Table 2. Four dif-
ferent processes may account for the consumption of trityl-
A similar argument allows the exclusion of the idea that
carboxylates act as general base catalysts (kcat). DABCO
and several quinuclidines have been reported to catalyze the
addition of water to tritylium ions in pure water, and it has
been shown that the logkcat values correlate linearly with
the corresponding pKaH values of the amines.[11b] Because
AcOꢀ is much less basic than DABCO, it should catalyze
the addition of water less efficiently than DABCO. The ob-
servation that AcOꢀ and BzOꢀ accelerate the consumption
of (MeO)3Tr + more than DABCO (Table 2, cf. slopes in en-
tries 1, 3, 4 or 2, 5) leads to the conclusion that carboxylate
ions attack the carbenium ion directly. These reactions are
reversible and do not occur in pure water, for which the
consumption rate of (MeO)3Tr + has been found to be inde-
pendent of the concentration of AcOꢀ.[6,7] Because the con-
centrations of AcOꢀ in the experiments of Figure 2 are con-
ACHTUNGTRENNUNGium ions in aqueous solvents in the presence of a base B
(carboxylate or amine). Apart from the reaction of the car-
bocation with water [kw in Scheme 2 and Eq. (4)], the alco-
siderably higher than they were during the ionization studies
[2]
ꢀ
of (MeO)3Tr OAc in the preceding paper, the equilibria
of Scheme 2 lie almost completely on the side of the cova-
lent ester. For this reason, in the experiments of Figure 2,
the absorbances of (MeO)3Tr + decreased mono-exponential-
ly to zero with kobs =kw +kꢀion[B], i.e., the slopes given in
Scheme 2.
entries 1 and 3 of Table 2 reflect kꢀion
.
hol can be produced by attack of hydroxide (kOH). Further-
more, the base can either act as a nucleophile and directly
attack the carbocationic center (k-ion), or catalyze the addi-
tion of water by abstracting a proton in a concerted manner
(kcat).
This rationalization of Figure 2 is supported by the follow-
ing considerations. Nucleophilicity parameters of benzoate
in 90AN10W have been determined as N25 =11.3 and s25 =
0.72 at 258C.[25] By employing Equation (3), the rate con-
stant for the bimolecular reaction of (MeO)3Tr + with BzOꢀ
can be calculated as 1.01ꢄ105 mꢀ1 sꢀ1. Comparison with the
experimental value of 2.32ꢄ104 mꢀ1 sꢀ1 (Table 2, entry 3) re-
veals agreement within a factor of 4.4, showing that the ob-
served rate constant is in the correct order of magnitude.
The deviation is within the range of tolerance of Equa-
tion (3), especially when sterically demanding electrophiles
like tritylium ions are involved.
ꢀd½Rþꢂ=dt ¼ ½Rþꢂðkw þ kOH½OHꢀꢂ þ kꢀion½Bꢂ þ kcat½BꢂÞ
ð4Þ
If the reaction with the base is reversible, a fifth term has
to be considered, which refers to the ionization of the
adduct (kion). In this case, Equation (4) transforms into
Equation (5).
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Chem. Eur. J. 2010, 16, 7478 – 7487

Electrophilicity versus Electrofugality of Tritylium Ions
FULL PAPER
An analogous calculation for the reaction of (Me2N)Tr+
with BzOꢀ yields a rate constant of 267mꢀ1 sꢀ1 [from
Eq. (3)], which is much bigger than the slope of 0.352mꢀ1 sꢀ1
given in entry 9 of Table 2, suggesting that this slope cannot
reflect the rate constant for the attack of BzOꢀ at
(Me2N)Tr+. Division of the larger of these two numbers
(kꢀion =267mꢀ1 sꢀ1) by kion (5.37 sꢀ1, from ref. [2]) yields the
upper limit for the equilibrium constant of the ester forma-
tion K=kꢀion/kion =50mꢀ1. Hence, for [BzOꢀ]=10ꢀ3 m (ꢃ20-
ꢀ
fold excess in our experiment), the ratio [(Me2N)Tr OBz]/
ACHTUNG-
TRENNUNG
[(Me2N)Tr+] must be smaller than 0.05, which implies that
the reaction of (Me2N)Tr+ with BzOꢀ in 90AN10W is ther-
modynamically unfavorable and, therefore, cannot be ob-
served. The small accelerations of the consumption of
(Me2N)Tr+ in the presence of carboxylates are concluded to
be due to the sum of kOH and kcat.
ꢀ
On the other hand, a ratio [(MeO)3Tr OBz]/
N
Complete free-energy diagrams for the hydrolyses of trityl
carboxylates: The ionization rate constants of trityl carboxy-
lates[2] and the rate constants for the consumption of trityl-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Figure 11. Free-energy profiles for the hydrolyses of substituted trityl
benzoates in 90:10 (v/v) acetonitrile/water, 258C; constructed for
[BzOꢀ]=1 molLꢀ1; [a] kw taken from Table 3, determined at 208C.
A
ACHTUNGTRENNUNG
combinations with n-nucleophiles by Equation (3). There-
fore, all of the quantities required for the construction of
free energy diagrams are now available.
In Figure 11, the covalent trityl benzoates are set on the
same level. The Eyring equation was used to calculate DGꢀ
values for the first step of the reaction cascade from directly
measured ionization rate constants kion (Table 8 in
ref. [2]).[26]
The rate constants for the combinations of the tritylium
ions with benzoate kꢀion in 90AN10W were calculated by
Equation (3) from the E parameters given in Table 1 and
the known nucleophilicity parameters of BzOꢀ (see above).
Thus, one arrives at the positions of the tritylium ions in
Figure 11. As discussed above, the directly measured rate
constant for the reaction of (MeO)3Tr + with BzOꢀ is 4.4
times smaller than that calculated by Equation (3). Taking
this factor into account would lower the positions of all tri-
tylium ions in Figure 11 by 3–4 kJmolꢀ1, a negligible correc-
tion in view of the total spread of the reactivities. The ther-
modynamic stability order of the tritylium ions (affinities
toward benzoate anions in 90AN10W), which has been ob-
tained on an entirely kinetic basis (kion and kꢀion), is in line
with the hydroxide affinity scale pKR+, which is based on
equilibrium measurements in water and aqueous sulfuric
acid. Figure 12 shows that the DG0 values derived from ki-
netic (kion and kꢀion) and equilibrium (pKR+) measurements
Figure 12. Correlation of free energies DG0 (combination of tritylium
ions with BzOꢀ, data taken from Figure 11) with DG0 from pKR+; slope=
1.07, R2 =1.00.
correlate with a slope of 1.07 with no obvious deviations.
The consistency of our kinetic data is thus confirmed.
The activation free energies for the last step in the reac-
tion cascade of Figure 11 can again be derived from the
Chem. Eur. J. 2010, 16, 7478 – 7487
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7485

H. Mayr and M. Horn
Eyring equation. Because the relative stabilities of trityl
benzoates and triarylmethanols are almost independent of
the nature of the substituents on the aryl rings, all energy
profiles will converge on the right of Figure 11. Readers
high intrinsic barriers encountered in reactions forming
(kion) and quenching (kw) these tritylium ions.
Apart from their use as protecting groups, tritylium ions
also have many practical applications as hydride abstracting
agents. This work has shown that there are excellent linear
correlations between electrophilic reactivities (logkw, E) and
the pKR+ values as well as the calculated hydroxide affinities
in the gas phase if the amino-substituted tritylium ions are
excluded. Parent, methyl-, and methoxy-substituted systems
can therefore be used as reference compounds for convert-
ing the manifold of published hydride abstraction rates by
differently substituted tritylium ions into a common activity
scale for hydride donors.[27]
ꢀ
should not be confused by the fact that Ar3C OH is located
ꢀ
at approximately the same level as Ar3C OBz. Like in
other ester hydrolyses, the equilibrium constants can be as-
sumed to be close to one and it is the high concentration of
water that is responsible for the almost quantitative hydroly-
ses.
Figure 11 can now be used to rationalize the kinetic phe-
nomena reported in this and the preceding article.[2] First of
all, one can recognize that the transition state of the ioniza-
tion step changes significantly as one goes from trityl ben-
zoate to the bis(dimethylamino)-substituted trityl derivative.
The more electron donors that are attached, the less carbo-
cation-like is the transition state. The origin of the observed
irregularities between methyl- and methoxy-substituted tri-
tylium systems on one side and dimethylamino-substituted
ones on the other is visualized well by Figure 11. As dis-
cussed earlier, the intrinsic barriers for the reactions of the
highly resonance-stabilized amino-substituted tritylium ions
are particularly high. As a consequence, the transition states
Experimental Section
Chemicals: Acetonitrile was used as purchased (VWR, 99.9%). Water
was purified by
a Millipore MilliQ device (final specific resistance
ꢄ18.2 MWcm). Tetra-n-butylammonium acetate (Fluka, ꢄ99%), tetra-n-
butylammonium benzoate (Fluka, ꢄ99%), and DABCO (Acros, 97%)
were used as purchased. Tritylium tetrafluoroborates as well as trityl ace-
tate were prepared according to synthetic procedures described previous-
ly.[2]
Kinetics: In all experiments the temperature was kept constant at 20 or
258C using a circulating water bath. Each measurement was repeated at
least once, the difference in kw not exceeding 3%. Reactions with half-
times greater than 10 s were followed by conventional UV/Vis spectrom-
etry using a J&M TIDAS instrument equipped with an insertion quartz
probe (Hellma). Faster reactions were studied with a stopped-flow UV/
Vis spectrometer (Applied Photophysics SX.18MV-R). For both tech-
niques the tritylium tetrafluoroborates were used as substrates, their ini-
tial concentrations ranging from 10ꢀ5 to 10ꢀ4 molLꢀ1. Poorly stabilized
carbocations reacting with half-lives below 10 ms were generated by
laser-flash photolysis, for which the corresponding trityl acetates were
used as precursors. The esters of MeTr, Me2Tr, Me3Tr, and (MeO)Tr were
generated by mixing equimolar amounts of the colored tritylium tetra-
fluoroborates and (nBu)4N+AcOꢀ in acetonitrile directly before the ki-
netic measurement, yielding colorless solutions. The laser pulse (7 ns
pulse width, 266 nm, 40–60 mJ/pulse) originated from an InnoLas Spit-
Light 600 Nd-YAG laser. Initial concentrations of trityl acetates ranged
ꢀ
ꢀ
for the ionizations of (MeO)3Tr OBz and (Me2N)Tr OBz
(68.9 and 69.7 kJmolꢀ1, respectively) as well as for the reac-
tions of the corresponding cations with water (93.8 and
96.0 kJmolꢀ1, respectively) almost coincide, although
(Me2N)Tr+ is the much better stabilized cation. Hence,
ꢀ
ꢀ
(MeO)3Tr OBz and (Me2N)Tr OBz ionize with similar
rates, whereas (Me2N)Tr+ reacts much more slowly with nu-
cleophiles than (MeO)3Tr +.
Conclusion
Although the linear free-energy relationship logk
(N+E) cannot generally be applied to reactions involving
sterically shielded systems (e.g., reactions of tritylium ions
with alkenes), we have now found that it works perfectly for
the decays of tritylium ions in aqueous acetonitrile. The
breakdown of the inverse correlation between the electrofu-
ACHTUNGTRNE(NUNG 208C)=
sACHTUNGTRENNUNG
from 10ꢀ4 to 10ꢀ2 molLꢀ1
.
Acknowledgements
ꢀ
galities of carbocations (rates of ionization of R X) and
their electrophilicities (rates of reactions of R+ with nucleo-
philes) for highly stabilized carbocations appears to be a
general phenomenon. As previously reported for the hydrol-
yses of benzhydrylium carboxylates,[25] we have now found
an excellent inverse correlation between the electrophilic re-
activities of methyl- and methoxy-substituted tritylium ions
We thank Prof. Shinjiro Kobayashi for assistance in using the laser-flash
apparatus and Dr. Armin Ofial for help during the preparation of this
manuscript. Financial support by the Deutsche Forschungsgemeinschaft
(Ma 673/20-3) and the Fonds der Chemischen Industrie is gratefully ac-
knowledged.
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much better stabilized in solution than expected from their
anion affinities in the gas phase (Figure 6), they must experi-
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strongly associated solvent molecules may account for the
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Chem. Eur. J. 2010, 16, 7478 – 7487

Electrophilicity versus Electrofugality of Tritylium Ions
FULL PAPER
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Received: September 28, 2009
Revised: February 24, 2010
Published online: May 21, 2010
Chem. Eur. J. 2010, 16, 7478 – 7487
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7487